The spread of information processing systems in recent years has been accompanied by greater demand for such systems to create many electronic documents in the form of color data and to output such data to color printers. Accordingly, it is desired that the image quality of color printers be improved.
A known method of improving the image quality of a color printer is to attach attribute bits to image data and execute the appropriate image processing. This method is such that at the stage where a raster image is generated in a host computer or printer, the type of drawing object (character, figure, image, color, monochrome, etc.) that constitutes the raster image is recognized, attribute bits indicative of the type of object are attached to each pixel constituting the object, and the raster data and attribute bits are adopted as the output data. When the output data is printed, the raster data is subjected to the appropriate image processing while reference is had to the attribute bits, whereby a printout having a high image quality can be obtained.
In the above-described method of adopting raster data and attribute bits as output data, the raster data and attribute bits are usually compressed and held in an output buffer. Since irreversible compression can be applied to the raster data at this time, size of the raster data after compression can be controlled by the compression rate. With regard to the attribute bits, on the other hand, the original data is necessary at the stage where image processing is executed and therefore only reversible compression is applied and size after compression is fixed. Consequently, there are occasions where, depending upon the type of data, size after compression is extremely large.
Accordingly, in a case where output buffer size is limited to a fixed value in order to limit system memory, the proportion of the buffer occupied by attribute bits increases if size after compression of the attribute bits is large. This means that the compression rate of the raster data must be raised to reduce the size of the raster data. If this is done, the decline in image quality ascribable to the increase in compression rate of the raster data will surpass the quality-improving effect achieved by use of the attribute bits. The result is a decline in the image quality of the output image.